PROJECT SUMMARY/ABSTRACT Breast cancer remains the most commonly diagnosed cancer in women and is the second leading cause of cancer-related deaths among women. Triple Negative Breast Cancer (TNBC) affects approximately 15-20% of all breast cancer patients, is the most aggressive sub-type of breast cancer and accounts for a disproportionately higher fraction of cancer-related morbidities and mortalities. Treatment options are extremely limited for TNBC patients and the most commonly employed neoadjuvant and adjuvant chemotherapy drugs have existed for decades. De novo and acquired chemotherapy resistance remains a major problem and disease recurrence results in breast cancer-related death for the large majority of patients. Further complicating the clinical management of TNBC is the lack of FDA-approved targeted therapies that can be utilized to prevent disease recurrence in the adjuvant setting. Thus, the identification and validation of novel drug targets and more effective treatment options continues to represent a major unmet clinical need. We have demonstrated that Estrogen Receptor Beta (ERβ) is expressed in approximately 20% of TNBCs, and have shown that patients with ERβ positive tumors have improved long-term prognosis. In addition, we have shown that ligand-mediated activation of ERβ by estradiol, or ERβ selective agonists, inhibits TNBC cell line and patient derived xenograft proliferation, invasion, and migration in vitro, as well as primary tumor growth and metastatic spread in vivo. Importantly, we provide the first evidence that estradiol can elicit clinical benefit in a patient with ERβ positive metastatic and chemo-refractory TNBC. Mechanistically, we demonstrate that ERβ potently suppresses the nuclear factor kappa B (NFκB) pathway in TNBC cells, effects that are mediated through association of ERβ with EZH2/PRC2 leading to epigenetic modifications to histone residues at NFκB target gene loci. Furthermore, we have demonstrated that ERβ modifies chemotherapy responsiveness of TNBC cell line models and patient derived organoids and inhibits chemo-resistant cell lines. Based on these data, the central hypothesis of this proposal is that ERβ repurposes EZH2 to inhibit NFκB signaling in TNBC thereby eliciting anti-cancer effects and enhancing chemotherapeutic responsiveness. To test this hypothesis, the following Specific Aims are proposed: 1). Determine the molecular mechanisms by which ERβ suppresses NFκB signaling in TNBC and 2). Elucidate the biological importance and clinical significance of ERβ-mediated suppression of NFκB signaling in TNBC using novel genetically engineered mice, PDX/PDO models and patient specimens. To conduct these Aims, we will utilize multiple model systems, innovative approaches and molecular tools, to comprehensively address our focused hypothesis. Given the extremely poor outcomes in women with TNBC, the proposed studies are of critical importance towards the goal of improving treatment strategies to mo...